1. Field of the Invention
This invention relates to the field of antiglare coatings in general, and in particular, to coating compositions, methods for preparing coating compositions and methods for applying coating compositions to optical glass surfaces. The invention is particularly directed to reducing specular reflection on the surfaces of cathode ray tubes, for example, computer monitor screens, television tubes, etc.
2. Description of Prior Art
Specular reflection or glare is defined as the direct reflection of ambient light from a smooth glass surface. Images on the screens of cathode ray tubes, for example television tubes, are formed behind the glass screen of the tube. Natural and artificial sources of light are reflected from the otherwise smooth glass surface of such screens, interfering with the images formed behind the glass surface. A strong source of sunlight, for example through an unshaded window, is likely to substantially wash out the entire picture. A local light source, for example a lamp, will tend to have its image reflected from the screen, superimposed on the image formed by the cathode ray tube (CRT). This creates a very disturbing local distortion.
Coatings have been applied to the surfaces of television tubes with a controlled roughness or surface pattern so that ambient light is scattered and diffused, thereby reducing glare. The roughness should not unduly degrade the resolution of the images to be viewed. A very practical consideration for coatings applied to CRT screens is that the glare-reducing coating should adhere to the glass surface, and should be sufficiently hard to resist abrasion and chemically resistant to moisture, humidity and common household cleaning solutions.
It is known in the art to reduce specular reflection with a vitrified, droplet pattern coating. Typically, an aqueous solution of an alkali silicate is sprayed in droplet form on a glass surface. The droplet pattern coating is dried and baked at an elevated temperature to provide a vitrified or glassy coating of corresponding pattern and surface contour. It is desirable to reduce the soda content in the vitrified coatings formed from such solutions in order to impart long-term stability against development of "bloom" on the coating surface. Such a solution is discussed in U.S. Pat. No. 3,114,668, which further teaches that picture or image resolution can be improved by incorporating a minor addition of boric oxide in the alkali silicate coating. Boric acid seemed to reduce the incidence of sharp-sided craters in the coating surface.
Another glare reducing coating is disclosed in U.S. Pat. No. 3,635,751, and is prepared by a method comprising the steps of: warming the surface of the glass screen to about 30.degree. C. to about 100.degree. C.; coating the warmed surface with an aqueous solution containing about 1 to 10 weight percent of a lithium-stabilized silica sol; drying the coating; and, heating the dry coating at about 150.degree. C. to 450.degree. C.
An improvement to the lithium silicate coating method is described in U.S. Pat. No. 3,940,511. It was observed that glare-reducing lithium silicate coatings on cathode ray tube face plates developed objectionable haze or "bloom" upon standing or storage at normal ambient humidities and temperatures. The haze is objectionable esthetically and reduces the brightness and color fidelity of the transmitted image. A similar haze was observed for sodium and potassium silicate coatings that have been baked at temperatures of about 400.degree. C. to about 500.degree. C. It was further observed that some glare-reducing lithium-silicate coatings which contained light attenuating particles transmitted an image which appeared to have a brownish or other tint. In the method according to the improvement, the dry baked coating is washed or rinsed with hot water subsequent to the baking step. Washing the coating with hot water reduces or eliminates the tendency of the coating to form a haze or bloom. The washing was believed to remove soluble lithium compounds which were present in the coating. In order to correct for any tint in the transmitted image which might be imparted by the glare-reducing coating, the coating might also include a small amount of a color-correcting dye.
Glare-reducing coatings are also of interest in applications other than glass screens, for example, on the surfaces of semiconductor solar cells. The object of anti-reflective coatings in this application is to promote transmission of, and to prevent reflection back into the atmosphere of solar radiation. Proper coatings can reduce the amount of light reflected when applied in thicknesses of one quarter of a wave length. Such coatings, as described in U.S. Pat. No. 4,361,598, can be made from clear solutions which contain oxide constituents in a soluble polymerized form and from which uniform and continuous glass-like oxide films can be deposited on substrates at relatively low temperatures. Such a solution is prepared by reacting metal alkoxide with a mixture of critical amounts of water and/or acid in an alcohol diluted medium. Alkoxides may be Ti(OR).sub.4 or Ta(OR).sub.5, or another metal alkoxide such as Si(OR).sub.4 in admixture with these alkoxides. Acids may be HCl or HNO.sub.3. Quarter wave inorganic optical coatings are deposited by applying the alkoxide solutions to a substrate and then heating the coating at a temperature above 350.degree. C. Of course, glare reducing coatings for such solar cells must be bounded to a surface of silicon doped with germanium, for example, which can be expected to react differently than glass in bonding with surface coatings.
Image quality can be difficult to measure objectively, particularly in evaluating resolution and contrast. Specular reflection has been customarily measured in terms of gloss or glare, objectively, by a gloss meter. Specular reflection can also be measured subjectively in terms of lines per inch and correspondence to a standard pattern. A series of patterns, having different numbers of lines per inch can be projected onto a test panel and reflected to a viewer. The last pattern capable of being distinguished is a valve or measure of the specular reflection.
The coatings and methods described herein are effective for producing anti-glare coatings on optical glass screens. The degree of glare reduction by coatings according to this invention has been determined both objectively and subjectively to be every bit as effective as the best known coatings of the prior art, and at the same time, can be prepared and applied at a significant cost savings. Accordingly, anti-glare coatings according to this invention provide very significant advantages over the prior art.